1. Technical Field
The present invention relates to a transmissive screen used in a rear projector, and particularly a heat resistant structure of the transmissive screen.
2. Background Art
Conventionally, in a rear-projection image projecting device using a projection tube or liquid crystal panel, namely in a rear projector, a transmissive screen is used that includes a Fresnel lens sheet having a lens with a saw-tooth cross section and a lenticular lens sheet having cylindrical lenses on its front and rear surfaces. In addition, transmissive screens having various structures are proposed in order to improve the contrast to outside light.
FIG. 11 is a sectional view showing a schematic structure of one example of a conventional transmissive screen. In FIG. 11, the conventional transmissive screen mainly includes Fresnel lens sheet 901, lenticular lens sheet 902, light diffusion layer 903, and transparent substrate 904. Projector 500 is installed behind the transmissive screen, and light from projector 500 is projected to the transmissive screen.
FIG. 12 is a partial sectional view showing a detailed structure of a conventional transmissive screen. In FIG. 12, the Fresnel lens sheet is omitted, and the lenticular lens sheet part that is particularly structured is enlarged.
As in FIG. 12, light diffusion layer 903 is formed by coating, by a printing method or the like, transparent substrate 904 with a material that is formed by dispersing light diffusion particulates into a binder made of a transparent material. Here, the refractive index of the particulates is different from that of a binder material. Lenticular lens sheet 902 is joined to light diffusion layer 903 with transparent adhesive 913 through light absorption layers (black stripe) 912. Light absorption layers 912 are disposed in land sections 911 having a substantially trapezoidal cross section on the emitting side of lenticular lens sheet 902.
When the outside light enters such a structure, the outside light diffused on light diffusion layer 903 is absorbed by light absorption layers (black stripe) 912 before a part of the outside light reaches a boundary with the air, and hence contrast reduction by the outside light is reduced. The phenomenon is prevented where a camber of an element causes a clearance between lenticular lens sheet 902 and light diffusion layer 903 to degrade the resolving power (for example, patent literature 1).
Recently, in a rear projector intended to be used as a digital advertisement pillar also in the outdoors, not only outside light contrast but also heat resistance to temperature increase by direct sunlight has been demanded.
In the conventional transmissive screen, however, the contrast to the outside light is improved by applying black stripe to the lenticular lens sheet. Therefore, when the transmissive screen is exposed to direct sunlight, the black stripe part absorbs light and hence the temperature of the lenticular lens sheet rapidly increases. Direct sunlight also increases the temperature inside a case of the rear projector, so that the temperature of the more inside elements is higher.
The Fresnel lens sheet and lenticular lens sheet are formed so that a specific cross-sectional shape can be formed at a predetermined accuracy. Therefore, the selection range of the material for them is small and it is difficult to use a transparent and heat-resistant material. The cross-sectional shape is complicated, so that this significantly influences the deformation by temperature increase. Thus, the conventional transmissive screen has a problem about heat resistance to direct sunlight.
When the rear projector is installed outdoors, the observation surface of the transmissive screen is exposed to the outside of the device. Therefore, the variety of installation conditions such as temperature/humidity or illumination is increased and unspecified person may touch the exposed part in various conditions, differently from the case where the projector is installed indoors. Therefore, it is important to secure the quality of the transmissive screen, such as strength against vibration or impact, weathering resistance, or visibility of a display image. Specifically, practical comprehensive measures are required which include measures against temperature increase inside the case by direct sunlight or the like, mechanical strength such as prevention of crack by vibration or impact, and measures against reflection of an image other than the display image.
Citation List
[Patent Literature]                [Patent Literature 1] Unexamined Japanese Patent Publication No. 2003-287819        